PCBs are open and can be downloaded on kh4's Github.
Those modules gives a 1200/2400 bauds serial link over audio channel.
1200 bauds seems retarded in 2014, but in digital RF data transmission, lower the baudrate is, greater the range will be.

Unfortunately native
communication protocols implemented in different flight controllers are usually fat babe not suited for low baudrates. They also mostly needs 2 way com ( will be hard with audio telemetry :) )

This is why I've added a lightweight protocol for this on Taulabs and Ghettostation.
I've almost finished the Baseflight implementation too ( Multiwii for STM32 fc like naze32 ).

This is a just first version of LTM ( LightTelemetry ) protocol, it will be improved later with more packet type in order to make a ground OSD too.

For now there's only one packet type sent every 200ms. Each packet has 18 bytes, giving 900bauds min.